Integrated circuits are commonly used in electronic devices to perform a variety of device operations, and are typically operated by a clock. Examples of integrated circuits include dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM) and other types of memory devices that may also include non-volatile memory devices. Integrated circuits, such as DRAM memory, are increasingly operated at faster clock speeds, which consequently increases power dissipation and temperature. Therefore, temperature sensors may be utilized, for example by a memory controller, to regulate the temperature of the device and improve memory performance at the higher temperatures. Examples of temperature sensors include a digital temperature sensor on chip with a continuous temperature readout capability. Temperature sensors, such as digital temperature sensors, are typically small so that the die size is not adversely affected, usually requires low power to operate, and are generally reliable without requiring frequent calibration, which minimizes test time overhead.
An example of a prior art digital temperature sensor 100 is shown in FIG. 1. The temperature sensor 100 utilizes a pair of diodes 16, 18 as the basic sensing element. The forward voltage drop across the diodes 16, 18 decreases linearly with temperature increasing. A current that is proportional to absolute temperature is generated (IPTAT) across a resistor 20 responsive to the diode 18 detecting the temperature. The PTAT voltage at node 21 is matched complementary to the diode voltage at node 23 and are provided as inputs to the amplifier 14 to subsequently enable the transistors 12a-c such that IPTAT may be appropriately sourced and adjusted through the diode 18. Similarly, a current that is complementary to absolute temperature (ICTAT) is generated through transistors 22a,b to cause the current ICTAT to flow through a resistor 26. Transistors 22a,b are enabled by another amplifier 24 that receives complementary input signals at node 23 and node 27, to adjust ICTAT responsive to changes to temperature. The IPTAT may be mirrored by the transistor 12c through a node 40 to charge a capacitor 32, or alternatively the ICTAT may be mirrored by transistors 28a,b to discharge the capacitor 32.
In operation, the capacitor 32, as it is being charged or discharged by either the IPTAT or ICTAT, is utilized to sample each of the currents responsive to the temperature being detected by the diodes 16, 18. A comparator (not shown) samples the voltage of the capacitor 32 and makes a decision and provides a digital output once every clock cycle relative to a clock signal being received. The capacitor 32 is sampled for “N” number of clock cycles, and the number of counts the comparator output is HIGH is recorded for the “N” number of clock cycles. The number of digital counts is then compared to a reference value that is used to determine a temperature readout.
A problem with the temperature sensor 100 is that there are a number of components in the circuitry that may be affected by device mismatch, which can negatively affect the accuracy of the temperature readouts. For example, the transistors 12a-c in block 11 should ideally be the same and have the same operating parameters to accurately generate the PTAT current proportional to temperature changes. However, as is well-known in the art, actual transistor components that are manufactured to be the same do not typically have exactly the same operating parameters. Device mismatch can also affect several other components in the temperature sensor circuit 100, such as amplifiers 14, 24. Ideally, the differential inputs to the amplifiers 14, 24 are matched and do not have any voltage or current offset so that the output signal can properly enable the corresponding transistors 12, 22. Even slight offsets in the amplifiers 14, 24 will affect how the respective IPTAT and IPTAT are sourced through the transistors 12, 22. Further mismatch may exist between transistors 22a,b, 28a,b in blocks 61, which will also create offsets that may affect the magnitude of IPTAT or ICTAT and adversely impact the accuracy of the temperature readout.
Conventional mismatch reduction techniques include utilizing a chopping amplifier or an auto-zero circuit. A chopping amplifier operates by switching the connections to the input terminals in synchronicity with switching of the connections to the output terminals in response to a clock signal. The switching results in cancellation of any voltage or current offset of the amplifier. Chopping amplifiers can be applied to the amplifiers 14, 24. A drawback to the chopping amplifier, however, is that the amplifier requires “settling time” to stabilize after each switching, which limits the operating speed of the temperature sensor 100. An auto-zero circuit utilizes a capacitor coupled to one of the inputs of the amplifiers 14, 24 to “store” the offset voltage which is used to provide an equal voltage offset and match the inputs. However, the auto-zero circuit typically requires a large capacitor to store the offset voltage, which can significantly increase the overall circuit layout area. Furthermore, both the chopping amplifier and the auto-zero circuit only improves performance of amplifiers 14, 24, and does not reduce effects of device mismatch of transistor components in blocks 11, 61.
There is, therefore, a need for a digital temperature sensor that is less susceptible to the adverse effects of device mismatch in the circuitry.